1. Field of Invention
This invention relates generally to a drawing device having a mobile stylus adapted to draw or trace on the undersurface of a glass plate rendered opaque by powder, various line drawings or sketches whose contours depend on the operator-controlled path taken by the stylus as it scrapes powder from the glass plate, and more particularly to a drawing device of this type in which the glass plate is so framed as to prevent the leakage of powder from the device, the glass plate being protectively shielded by a transparent rigid or semi-rigid film of heat-formable material.
2. Status of Prior Art
French Pat. No. 1,242,870, and the Grandjean U.S. Pat. No. 3,305,113, as well as the Clark U.S. Pat. No. 3,760,505, disclose a tracing device having educational as well as play value. The tracing device includes a box-like case having a transparent top glass plate or screen next to which are left and right control knobs. By turning these knobs, one can delineate on the screen by means of a mobile stylus various letters, charts, designs and other line drawings and sketches.
In a drawing or tracing device of this type, in order to form a horizontal line on the X-direction on the screen, the operator has only to turn the left knob, while to form a vertical line in the Y direction, he turns the right knob. And to create curves and angles on the screen, these knobs are turned at the same time, thereby causing the stylus to move in a path which is the vector resultant of the X and Y movements.
The case is partly filled with a slightly adhesive powder which sticks onto the undersurface of the screen to render it opaque but is easily dislodged therefrom. The undersurface of the screen is engaged by the mobile stylus which under knob control scrapes the powder from the screen to define a line whose contour depends on the operator-controlled path taken by the stylus.
To erase the line drawing or sketch defined by the scraped-off powder on the screen, the case is turned upside down and is shaken to cause the powder to cascade over the undersurface of the screen, to again form an opaque coating.
The fragility of the glass plate is a potential danger, especially for children. For this reason, safety standards dictate the use of a protective film made of synthetic material to cover the outside of the glass plate.
In order to shield the glass plate, the above-identified Clark patent imprisons a flexible plastic film under an outer frame which is joined to the housing of the tracing device and surrounds the window. It is difficult, however, especially in large scale production of such drawing devices, to place this flexible film in its proper position and to fix it in place without at the same time creasing the film. Such creasing interferes with the optical properties of the drawing device.
Another drawback of the drawing device disclosed in the Clark patent is that in order to prevent leakage of the powder from inside the device, a non-hardening adhesive must be applied between the marginal surface of the glass plate and an inner liner or side wall support therefor which includes a groove for the adhesive. These expedients add to the production difficulties and the costs entailed in large scale manufacture of the drawing device.